Protein synthesis in yeast. Identification of an altered elongation factor in thermolabile mutants of the yeast Saccharomyces cerevisiae.

Cell-free extracts from the wild type yeast strain (A364A) and from a group of noncomplementing mutants that are conditionally defective in translation were preincubated at a restrictive temperature prior to incubation at a permissive temperature for protein synthesis. Results of these experiments showed that upon exposure to the restrictive temperature (39 degrees C), all five of the noncomplementing mutants lost ability to incorporate amino acid into protein. The wild type parent strain retained better than 80% of the activity under identical conditions of heat treatment. Mutant extracts could be revived to incorporate amino acid by the addition of the purified yeast elongation factor 3. Factors 1 and 2 had no effect. The heat-treated extract from one mutant did not supplement the activity of the other mutant. Although all five of the mutants were inactivated by preincubation at 39 degrees C, each showed a variable rate and extent of thermolability. Heat-treated mutant extracts were fully active in polyphenylalanine synthesis with liver ribosomes but not with the yeast ribosomes. Since liver ribosomes do not require factor 3, this assay then confirms that factor 3 is the thermolabile component in this group of noncomplementing mutants.